Weathering characteristics of some soils formed in White River Tephra, Yukon Territory, Canada

Soil formation under a cold, dry climate on two Late Holocene (approximatel y 1800 yr BP and 1150 yr BP) volcanic tephra deposits was examined to evalu ate trends and rates of soil weathering. Presently under mature boreal fore st, these soils are underlain by permafrost within 1 m of the soil surface. The soils are composed of buried forest floor materials acid layers of coa rse ashy-pumiceous rhyodacite tephra up to 60 cm thick. The bulk density in all tephra-derived horizons is less than 0.90 Mg m(-3), water retention va lues for these horizons at 1500 kPa suction are less than 10% by weight. Th e mineralogy of the tephra-derived horizons is dominated by volcanic glass with lesser amounts of plagioclase, microcline and amphibole. Micromorpholo gical investigation showed minor pitting of glass and feldspar grains and s ome oxyhydroxide coatings and Fe and Al-humus complexes. Several soil horiz ons meet glass composition, P retention and oxalate-extractable Fe and Al c riteria established for andic soil material. However none of the profiles m eet the thickness requirement to be classified as Andisol as specified in S oil Taxonomy. It is estimated that it would take >4000 yr under present cli matic conditions to achieve such a degree of weathering.